[Core] Refactoring sampler and support prompt logprob for chunked prefill

[rkooo567]

Summary;

Refactoring sampling metadata and sampler. More concretely

• Introduce SequenceGroupToSample class instead of having multiple list data structure to combine
• Instead of writing index looping logic again and again, we prepare the prefill/decode indices ahead of time and reuse it
• Move prepare_sample to SamplingMetadata
• Remove all indexing logics that assume requests are entire prefill or decode
• Introduce do_sample to SequenceGroupMetadata. If it is set to False, sampling/sample logprob calculation for the corresponding seq_group is skipped.
• Improve docstring and confusing variable names
• Remove perform_sampling because it is leaky and overlaps with do_sample. I just use is_driver_worker directly for the same purpose.

Fix prompt logprob for chunked prefil

• Existing logic has strong assumption where the seq group only contains entire prefill or decode.
• Allow to incrementally update prompt logprobs
• Allow to skip sampling when it is chunked prefill (since it is not required)

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[rkooo567]

cc @cadedaniel is this assumption correct?

[cadedaniel]

correct!

[rkooo567]

Will remove after confirming tests are passed

[rkooo567]

Will remove after confirming tests are passed

[zhuohan123]

Thank you for the refactor SangBin! The code looks much better now. I left some small comments. But in general the code looks pretty good to me.

[zhuohan123]

seq_group_idx -> seq_group_ids? Originally I would like to emphasize this is a list of IDs.

[rkooo567]

reverted to id. I thought it was confusing because each seq_group already has request_id (and it doesn't match). But no strong preference.

[rkooo567]

@zhuohan123 thanks for the review! All comments are addressed!

[rkooo567]

yay. Thanks for the review again @zhuohan123 !! time to refactoring model runner...